1. Field of the Invention
The present invention generally relates to an optical disk apparatus compatible with recording mediums (hereinafter, referred to as optical disks) of different base thickness and to an optical disk apparatus in which a laser beam is effectively used and the size of the apparatus is reduced.
2. Description of the Related Art
FIG. 1 is a schematic diagram showing the construction of an optical disk apparatus according to the related art. Referring to FIG. 1, a linearly-polarized laser beam emitted by a semiconductor laser 101 is transformed into a parallel beam by a collimating lens 102. The collimated light beam is caused to pass through a polarizing beam splitter 103 and transformed into a circularly-polarized beam by passing through a .lambda./4 plate 104. The laser beam is then deflected by a deflecting prism 105 so as to be incident on an object lens 106. The objective lens 106 converges the light beam so that a laser spot of a small diameter is formed on an optical disk 107.
The laser beam reflected by the optical disk 107 is transformed into a circularly-polarized beam polarized in a direction opposite to the beam on an upstream path incident on the optical disk 107. The reflected light beam is transformed into a parallel light beam by the objective lens 106. The parallel beam is then deflected by the deflecting prism 105 and passes through the .lambda./4 plate 104. By passing through the .lambda./4 plate 104, the laser beam is transformed into a linearly-polarized beam polarized in a direction perpendicular to the polarizing direction of the laser beam on the upstream path. The linearly-polarized beam is incident on and reflected by the polarizing beam splitter 103. The reflected light beam is converged by a converging lens 108. The converged beam is incident on a photosensitive element 109. An data signal and a servo signal are retrieved based on a signal from the photosensitive element 109.
The background of the present invention is that there is a great demand for large-capacity optical recording. In order to meet such a demand, efforts are being made to reduce the wavelength of the laser beam.
Generally, the diameter of the laser beam spot formed on the optical disk 107 is proportional to the wavelength of the laser beam. The recording capacity increases in proportion to the square of the wavelength. Accordingly, the recording capacity is increased by reducing the wavelength of the laser beam.
However, it is to be noted that the reflectivity of the optical disk 107 and the required write power for the optical disk 107 may depend largely on the wavelength. When a laser beam of a short wavelength is used in the optical disk 107 characterized by a large degree of dependency, reading and writing may be disabled. That is, the optical disk apparatus may fail to be compatible with optical disks of different types.
One approach to overcome this problem is to use two laser beams, one having a conventional wavelength (for example, 785 nm) and the other having a wavelength (for example, 650 nm) smaller than the conventional wavelength. A simple way of implementing this approach is to provide two optical pickups by using two laser sources emitting two laser beams having different wavelength and two objective lenses having respective optical characteristics adapted for the different wavelength.
However, the use of two optical pickups increases the size and cost of the apparatus. Japanese Laid-Open Patent Application No. 6-259804 avoids this drawback by constructing an optical disk apparatus using a single optical pickup having two laser beam sources and one objective lens.
In a construction where two laser beam sources and one objective lens are provided, the quantity of light of the laser beam emitted by the laser source is detected by an actinometer using a split laser beam. However, it is difficult to split the laser beams from the two laser sources so that the two laser beams have an equal quantity of light or to split the laser beams with predetermined ratios of quantity of light assigned to the respective beams. For this reason, gain control should be performed when the signal from the actinometer is processed. There is a problem in that provision of a gain control circuit increases the cost of the apparatus.
Further, since it is difficult to control the quantity of light of the split beams properly, there is a problem in that it is difficult to ensure that the laser beam having an appropriate quantity of light is incident on the actinometer so that the laser beams are effectively used.
An additional problem is that, when the upstream optical path is split from the downstream optical path, the upstream optical path is perpendicular to the downstream optical path, thus making it difficult to reduce the size of the apparatus.
A description will now be given of another aspect of the related art to which the present invention is applied.
Recently, there is a growing demand for large storage capacity in an optical recording medium such as an optical disk. In order to increase the storage capacity without increasing the medium itself, the diameter of a light beam spot for writing and reading of information should be increased. Since the diameter of a light beam spot is proportional to a square of a wavelength .lambda., the storage capacity is inversely proportional to the wavelength .lambda.. For this reason, reduction of the wavelength of a laser beam used in an optical pickup apparatus is sought. While a wavelength of 785 nm is used for writing and reading of information in a conventional CD-R optical disk, a reduced wavelength of 650 nm is used for a DVD optical disk now available.
Japanese Laid-Open Patent Application No. 6-259804 discloses an optical pickup apparatus in which two types of semiconductor lasers (referred to as light sources (LD) in the specification) emitting laser beams of difference wavelength so that the apparatus can be used for optical recording mediums characterized by different operating wavelength for writing and reading of information.
As is well known, a light beam emitted by a light source LD is divergent, the angle of divergence being maximum in a direction perpendicular to an active layer and minimum in a direction parallel with the active layer so that a far field pattern is elliptical in its configuration.
It is preferable that a light beam spot formed on an optical recording medium is circular. As the light beam spot approaches an elliptical configuration, the writing and reading performance becomes unfavorable.
In order to obtain a circular light beam spot, a portion of a light beam from the light source LD at the extremes of the major axis of a far field pattern may be blocked before the light beam is incident on a coupling lens for coupling. In this way, a circular beam section is obtained. However, according to such a method, not a small portion of the light beam from the light source is blocked, resulting in an poor efficiency in using optical energy for writing and reading. Since the optical energy required in an optical pickup apparatus for writing on an optical recording medium is more than ten times the energy required for reading, it is preferable for a maximum portion of the light beam from the light source to be exploited to form a light beam spot for proper information writing.
Merely exploiting the edge portions along the major axis of the far field pattern results in an elliptical configuration of a light beam spot (the minor axis of the far field pattern corresponds to the major axis of the light beam spot). In addition to exploit the edge portions along the major axis of the light beam spot, a beam shaping action is performed so that the section of the light beam approaches a circular configuration.
Beam shaping is implemented by a combination of two prisms or a cylindrical lens. However, using prisms or a cylindrical lens may increase the size of the apparatus.